Apparatus for detecting a rotation angle of a diffraction grating

ABSTRACT

In order to improve the accuracy of the absolute value of a wavelength of diffraction light in a diffraction grating, a gas absorption line resulting from an absorption cell 8 is used as a wavelength reference 8. When reference light is exited from a light source 7 in a wavelength reference light source 1, the reference light is transmitted to a diffraction grating 2 as transmitted light having a spectrum absorbing only light of a predetermined wavelength by the absorption cell 8 to allow it to be reciprocated in a predetermined angle range. The diffraction grating 2 produces a split light beam from the transmitted light from the absorption cell 8. The diffraction grating 2 splits the transmitted light from the absorption cell 8 to provide diffracted light and the diffracted light from the diffraction grating 2 is received by the reference light receiving unit 3. It is possible to, without being affected by a variation in the environmental condition, accurately know the rotation angle of the diffracting grating 2 from the diffracted light of the absorption line-existing waveform component received by the reference light receiving unit 3, that is, the rotation angle of the diffraction grating 2 at a wavelength at that time.

TECHNICAL FIELD

The present invention relates to an apparatus for detecting a rotationangle of a rotatable diffraction grating generally used for an opticalapparatus and, in particular, an apparatus for detecting a rotationangle of a diffraction grating applicable to an optical spectrometerapparatus (optical spectrum analyzer), a tunable wavelength lightsource, etc.

BACKGROUND ART

The diffraction grating used in the field of an optical apparatus isrotatable as well known, has a great number of grooves provided on itssurface in a predetermined interval, and is composed of an element forgenerating diffracted light due to an interference of light reflected atthose smooth surfaces between grooves in accordance with an incidentangle of light.

As an optical apparatus using such diffraction grating there has beenknown a light spectrum analyzer as shown in FIG. 8 (see AnritsuTechnical Review No. 13, July 1991, pp 36 to 49).

When a wavelength distribution, etc., of light under consideration ismeasured in the optical spectrum analyzer, a diffraction grating 31 hasits rotation controlled by a motor 33 while its rotation angle isdetected by an angle detector 32, such as an encoder, potentiometer,etc.

The light to be measured is split by the diffraction grating 31.

The exit light split by the diffraction grating 3 is narrowed down, by aslit 34 in a slit mechanism 34, on a focusing position and then receivedby a light receiving unit 35 and displayed as spectra on a display unit,not shown.

Upon the measurement of the waveform distribution (spectra) of the lightto be measured in this kind of a light spectrum analyzer, the absolutevalue of the waveform relative to the to-be-measured light is firstfound from a theoretical diffraction angle inherent in the diffractiongrating 31.

The rotation angle of the diffraction grating 31 is controlled bysetting the absolute value of the waveform so as to correspond to thepulse number of an encoder as the angle detector 32 or to a voltage ofpotentiometer. Stated in more detail, a number is allocated to a pulseoutput from the encoder as the angle detector 32, or to a voltage of thepotentiometer, for each predetermined angle in a rotatable angle rangeof the diffraction grating 31, so that the absolute value of thewaveform is allocated to the corresponding number.

FIG. 9 shows the diffraction order of the diffracted light by thediffraction grating 31 of this kind and its diffraction.

For example, let it be assumed that light of wavelength 1 μm is incidentat an angle of incidence of 15° with respect to a normal N. Then thelight is diffracted in 10 directions, that is, with the order ofdiffraction, m=-3, -2, -1, 0, 1, 2, 3, 4, 5, 6.

The light receiving unit 35 of the light spectrum analyzer as shown inFIG. 8 receives diffracted light of the order m=1, 2.

In this case, the angle of diffraction, βm, of the incident light isfound from the following equation (1).

    mλ=d cos θ(sin i+sin βm)                 (1)

where

d : groove-to-groove interval of the diffraction grating;

i : angle of incidence;

θ: angle of the incident light made with respect to an XY plane;

λ: absolute wavelength of the incident light; and

m : order of diffraction. (m=0, ±1, ±2, . . . )

Further, a tunable wavelength light source is known as an optical unitutilizing this kind of diffraction grating (see Optical FiberTelecommunications II, pp 533 to 536, Academic Press, Inc. 1988).

This tunable wavelength light source is such that exit light from alaser diode 36 is reciprocably moved back and forth relative to adiffraction grating 31 and resonated light is output from the other endof the laser diode 36.

Such an external cavity type tunable light source is such that thewavelength of output light from the other end of the laser diode 36 isvariably controlled by varying the rotation angle of the diffractiongrating 31 by means of a motor 33.

Like the above-mentioned optical spectrum analyzer, the diffractiongrating 31 has its rotation controlled by the motor 33 while rotationangle is detected by an encoder and an angle detector 32 such as apotentiometer.

In the above-mentioned optical spectrum analyzer and tunable wavelengthlight source, however, if the environmental condition, such as theambient temperature, humidity and pressure, varies, then the diffractionangle of the diffracted light from the diffraction grating 31 variesbecause the refractive index of air as well as the groove-to-grooveinterval of the diffraction grating 31 delicately varies.

Due to an adverse effect resulting from the variation of theenvironmental condition, the absolute value of the wavelength ofdiffracted light from the diffraction grating 31 is not always inagreement with the theoretical value and varies, thus lowering theaccuracy in the measurement of the waveform. In the above-mentionedoptical spectrum analyzer and tunable wavelength light source,therefore, it is not possible to measure a stable wavelengthdistribution at all times and output light of a desired wavelength.

For example, let it be assumed that, as the environmental condition, thetemperature of 25° C., humidity of 50% and atmospheric pressure 1 hPaare provided as a reference. If, in this case, the environmentalcondition varies in a range of 5° to 45° C. in temperature, 0 to 90% inhumidity and 0.95 to 1.05 hPa in atmospheric pressure, the accuracy withwhich the wavelength is measured by the optical spectrum analyzer is 0.5nm and the accuracy of the waveform of exit light from the tunablewavelength light source is 0.1 nm.

As the method for securing these accuracies it is considered that theoptical spectrum analyzer and tunable wavelength light source are heldin a thermostatic and in a vacuum container for management. This makesan apparatus bulkier as a whole and is not practical.

That is, this problem arises from the fact that it is not possible toaccurately detect the rotation angle of the diffraction grating becauseit is affected by a variation in the environmental condition.

DISCLOSURE OF INVENTION

The present invention has been conceived with consideration given to theabove-mentioned problem and pays attention to the fact that, if, as awavelength reference, use is made of a gas absorption line not affectedby the absolute values of a waveform even under a variation in theenvironmental condition it is possible to accurately detect the rotationangle of a diffraction grating and hence to pick up accurate diffractionangle data at all times from the diffraction grating and an object ofthe present invention is to provide an apparatus for detecting arotation angle of a diffraction grating which is applicable to anoptical spectrum analyzer capable of improving the accuracy of theabsolute values of a wavelength of diffracted light in the diffractiongrating and effecting high-accuracy measurement and a tunable wavelengthlight source capable of exiting light of a stable, desired wavelength.

According to one aspect of the present invention there is provided anapparatus for detecting a rotation angle of a diffraction grating whichincludes a rotatable diffraction grating for receiving incident lightand exiting split beams, a drive unit for rotating the diffractiongrating, and an angle detecting unit for detecting a rotation angle ofthe diffraction grating, comprising:

a light source unit including a light source and absorption cell and forexiting, to the diffraction grating, reference light of a regionincluding a specified wavelength determined depending upon theabsorption cell;

a reference light receiving unit for receiving a split reference lightfrom the diffraction grating and converting the split reference lightinto an electric signal; and

a signal processing unit for detecting an extreme value of the electricsignal from the reference light receiving unit by rotating thediffraction grating by the drive unit, receiving a rotation angle outputfrom the angle detecting unit when the extreme value is detected, andcalculating a specified rotation angle corresponding to the specifiedwavelength.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic view showing an arrangement of an apparatus,according to a first embodiment of the present invention, for detectinga rotation angle of a diffraction grating;

FIG. 2 is a schematic view showing an arrangement of an apparatus,according to a second embodiment of the present invention, for detectinga rotation angle of a diffraction grating;

FIG. 3 is a schematic view showing on optical spectrum analyzer as anoptical spectrometer apparatus to which the present invention isapplied;

FIG. 4 is a schematic view showing a tunable wavelength light source towhich the present invention is applied;

FIG. 5 is a schematic view showing an optical spectrum analyzer to whichthe present invention is applied;

FIGS. 6A and 6B are views showing the correction of substantialgraduations on a display screen on the basis of correction data;

FIG. 7 is a flow chart for explaining a reference position measuringprinciple;

FIG. 8 is a schematic view showing a conventional optical spectrometerapparatus;

FIG. 9 is a view showing the order of diffraction light by a diffractiongrating and its directions; and

FIG. 10 is a view showing a conventional tunable wavelength lightsource.

BEST MODE OF CARRYING OUT THE INVENTION

The embodiments of the present invention will be explained below withreference to the accompanying drawings.

FIG. 1 is a diagrammatic view showing a diffraction grating's rotationangle detecting apparatus according to the present invention.

The rotation angle detecting apparatus shown in FIG. 1 comprises awavelength reference light source 1, a diffraction grating 2, areference light receiving unit 3, an angle detector 4, a drive unit 5and a calculation unit (signal processing unit) 6.

The wavelength reference light source 1 comprises a light source 7 andan absorption cell 8.

The light source 7 is comprised of a white color light source, or an SLD(superluminescent diode), for outputting light containing variouswavelengths, so as to output a light beam of a predetermined wavelength.

The absorption cell 8 is comprised of a tube sealed with a gas having anabsorption line absorbing the light of a predetermined wavelength at alltimes without being affected by a variation in the environmentalcondition.

For example, as a gas possessing an absorption line in a 1.5 μm bandthere are C₂ H₂, CO₂, NH₃, H₂ O, HCN, CH₃ Cl, etc.

Further, as a gas possessing an absorption line in a 1.3 μm band, thereare CH₄, NH₃, H₂ O, HF, etc.

As a gas possessing an absorption line in a 0.8 μm band there are H₂ O,Rb, Cs, etc.

As documents for these gas absorption lines, there are, for example,Frequency Stabilization of Laser Diode Using 1.51-55 μm Absorption Linesof ¹² C₂ H₂ and ¹³ C₂ H₂, IEEE Journal of Quantum Electronics, Vol. 28,No. 1, January 1992, pp 75 to 81 and Frequency Stabilization of AlGaAsSemiconductor Laser Based on the ⁸⁵ Rb-D₂ Line, Japanese Journal ofApplied Physics, Vol. 21, No. 9, September, 1982, pp L561-L563, etc.

The absorption cell 8 using such a specified gas allows light from thelight source 7 to exit to the diffraction grating 2 as transmitted lightpossessing a spectrum absorbing only a given waveform component, thatis, a component having an absorption line inherent in that specifiedgas.

In this case, as set out above, the wavelength reference light source 1uses the white color light source or SLD and its output light possessesa few absorption lines.

For the absorption cell 8 using, for example, a C₂ H₂ gas, theabsorption line exists for each wavelength of about 0.9 nm to 1 nm.

The half value width of the absorption line differs depending upon thepressure (density) of the sealed gas in the absorption cell 8.

The pressure of the sealed gas is adjusted with respect to theabsorption cell 8 in accordance with a measuring resolution of thediffraction grating 2. For the absorption cell 8 using a C₂ H₂ gas thehalf value width of about 1 GHz is obtained under a sealed pressure of100 Torrs.

The diffraction grating 2 has a greater number of grooves 2a provided inthe surface at predetermined intervals and the transmitted light comingfrom the absorption cell 8 side is reflected on a smooth surface betweenthe grooves 2a toward the reference light receiving unit 3 side (seeFIG. 9).

The reference light receiving unit 3 receives the diffracted light fromthe diffraction grating and an electric signal corresponding to thatreceived light amount is output to the calculating unit 6.

The angle detector 4 detects the rotation angle of the diffractiongrating 2 and is comprised of an encoder for outputting a pulse eachtime the diffraction grating 2 is rotated at a rate of a predeterminedangle, a potentiometer for developing a voltage due to a variation of aresistive value resulting from the rotation of the diffraction grating2, and so on.

Stated in more detail, with the diffraction grating 2 whose rotation iscontrolled in a range of, for example, 0° to 40°, the calculation unit 6attaches a number to a pulse output from the encoder as the detector 4,or to a voltage at the potentiometer, for each predetermined angle of arange 0° to 40°, though being not illustrated, and allocates theabsolute value of the wavelength to the number.

In the range of, for example, 0.6 to 1.75 μm for the later-describedoptical spectrum analyzer and in the range of, for example, 1.4 to 1.7μm, correspondence is taken by the calculation unit 6 to the wavelengthof the transmitted light from the absorption cell 8.

The drive unit 5 is equipped with a motor and, when the wavelength isset by the operator via the calculation unit 6 though being notillustrated, the motor is driven in accordance with a table on thecalculation unit so that the diffraction grating 2 may be rotated to aposition to which this wavelength is allocated.

The drive unit 5 drives the diffraction grating 2 in normal/reverserotation so as to perform scanning in a predetermined angle range (forexample, 0° to 40) with a horizontal state as a reference as indicatedby a dot-dash line.

Although being omitted in illustration, the drive unit 5 drives themotor, after a later-described correction, on the basis of thecorrection data from the calculation unit 6 to cause the diffractiongrating 2 to be driven in rotation motion.

The above-mentioned calculation unit 6 is comprised of a microcomputerequipped with, for example, a CPU, ROM and RAM and previously stores, inthe form of a table, the rotation angle data of the diffraction grating2 corresponding to the wavelength of the light to be measured and data,such as a voltage value, of the potentiometer as the angle detector 4corresponding to the width of the wavelength.

The calculation unit 6 performs predetermined signal-processing forcalculating the rotation angle (correction data) of the diffractiongrating 2 on the basis of the input data, such as the wavelength set bythe operator, electric signal from the reference light receiving unit 3,and angle detection signal from the angle detector 4.

The calculation unit 6 calculates, as the rotation angle of thediffraction angle 2 (for example, the angle with the horizontal positionas the reference), a correction amount corresponding to a displacementrelative to an angle detection signal (the pulse number of the encoderor voltage of the potentiometer) of the angle detector 4 given anallocated wavelength of the to-be-measured light, though being omittedin illustration, based on the absolute value of the wavelength of anabsorption line when the reference light receiving unit 3 receives thediffracted light of a wavelength component with that absorption linepresent therein.

In the apparatus for detecting the rotation angle of the diffractiongrating thus structured, when the light is incident on the absorptioncell from the light source 7, transmitted light having a spectrum, thatis, a spectrum absorbing only a wavelength component with the absorptionline present therein, exits form the absorption cell 8 without beingaffected by a variation in the environmental conditions.

The diffraction grating 2 receives the transmitted light and thereference light receiving unit 3 receives it as diffracted light fromthe diffraction grating 2.

Through the utilization of a variation in level of the diffracted lightof the diffraction grating 2 produced due to each absorption line in theabsorption cell 8 (λ1, λ2 in FIG. 1) differing in absorptivity, thecalculation unit 6 identifies the diffracted light, that is, lightreceived by the reference light receiving unit 3, as emerging due to theabsorption line of any specified wavelength and calculates the rotationangle of the diffraction grating 2.

Stated in more detail, the calculation unit 6 compares the pulse number,that is, a pulse number of the encoder as the angle detector 4, or avoltage value of the potentiometer, with the tabled data when thereference light receiving unit 3 receives the diffracted light andcalculates the rotation angle of the diffraction grating 2 correspondingto the wavelength of the light at that time.

Here, explanation will be given below of the measuring principle forcalibrating the wavelength data previously stored in table form.

First, this measurement is carried out with the use of the followingequation (2).

    mλ=A sin θ

where

m : diffraction order of the diffracted light;

λ: wavelength

A : constant of the optical system

θ: displacement angle with respect to an origin corresponding to theposition (angle) of the diffraction grating when 0-order light isreceived.

In the equation (2), the constants to be found are the constant A of theoptical system and displacement angle θ.

Here, in order to find the displacement angle θ, it is necessary toobtain a count value of the encoder, as the angle detector 4, allocatedto the 0-order light origin.

If the count value of the encoder at the 0-order light's origin andconstant A of the optical system are found with the reference light, itfollows from the equation (2) that it is possible to find a relation ofthe wavelength λ and count value of the encoder 4.

Stated in more detail, in the structure shown in FIG. 1, reference lightof a wavelength λ1 is input from the light source 7 to the diffractiongrating 2 as indicated in a flow chart of FIG. 7--step S1.

At step S2, the power of the diffracted light obtained by the rotationof the diffraction grating 2 is measured by the reference lightreceiving unit 3 and calculation unit 6.

It is decided, at step S3, whether the power of the diffracted light ismaximal or not.

When, by that decision, the power of the diffracted light becomesmaximal, the diffracted light at that time is regarded as correspondingto the 0-order light and the count value (a) of the encoder 4 is storedat step S4.

Then, with the use of the 1-order light of the diffracted light thediffraction grating 2 is moved to a position of a wavelength λ1 on thebasis of the waveform data previously stored--step S5.

At step S6, the power of the diffraction grating 2 is measured byrotating the diffraction grating 2.

Stated in more detail, at this time, the diffraction grating 2 isrotated in a wavelength range of about ±0.7 nm with the position of thewavelength λ1 as a reference.

Then at step S7, it is decided, this time, whether the power of thediffracted light is minimal or not.

When the power of the diffracted light is minimal, the diffracted lightat that time is regarded as corresponding to the waveform λ1 and thecount value (b) of the encoder 4 is stored at step S8.

At step S9, the sin θ in equation (2) is found from the count values(a), (b) stored as set out above and the constant A of the opticalsystem is found from the sin θ, m=1 and λ=λ1. Based thereon, thewavelength is rewritten as the wavelength data.

In the apparatus, according to the present invention, for detecting therotation angle of the diffraction grating, as set out above, use ismade, as a waveform reference, of the gas absorption cell unaffected bythe variation of the environmental condition and, without being affectedby the variation of the environment such as the atmospheric temperature,humidity and pressure, it is possible to achieve an improvement in theabsolute value of the wavelength of the diffracted light coming from thediffracted grating 2 and to also accurately detect the rotation angle ofthe diffraction grating 2 with respect to the wavelength at that time.

The absorption line can also be determined, in place of from thedifference in absorptivity of the respective absorption lines, from aninterval (Δλ in FIG. 1) present in the absorption line in which caseattention is paid to the interval present at the absorption line.

FIG. 2 is a schematic view showing an apparatus, according to thepresent invention, for detecting a rotation angle of a diffractiongrating.

The rotation angle detection apparatus shown FIG. 2 is of such a typethat a waveform stabilizing light source 11 is used in place of thewaveform reference light source of a structure as shown in FIG. 1. Theremaining part of the apparatus is the same as that of FIG. 1 and anyfurther explanation thereof is omitted.

Usually, the output light of a laser diode single unit used as the lightsource has its center wavelength varied due to the variation of theenvironmental conditions.

In order to solve this problem, the wavelength stabilizing light source11 comprises, in addition to a light source 12 comprised of a laserdiode for outputting light of a predetermined wavelength, a lightsplitting unit 13, absorption cell 8, light receiving unit 14 forwaveform control, and wavelength control circuit 15 and allowsstabilized laser light to exit to a diffraction grating 2.

The light splitting section 13 is comprised of, for example, a beamsplitter, photocoupler, etc.

The light splitting section 13 allows laser light which comes from thelight source 12 to be split in two directions.

Of those split light beams, one light beam is incident on the absorptioncell 8 for absorbing light of a specific wavelength and the other lightbeam is incident on the diffraction grating.

The light receiving unit 14 for wavelength control receives a light fromthe light source 12, as a light beam transmitted through the absorbingcell 8, and converts it to an electric signal and outputs the electricsignal to the waveform control circuit 15.

The wavelength control circuit 15 receives an electric signal from thelight receiving unit 14 and controls the light source 12 so that thestrength of the transmitted light beam coming via the absorption cell 8from the light source 12 becomes constant. By doing so, it is possibleto make constant the wavelength of the light emerging from the lightsource 12.

Thus, the waveform stabilizing light source 11 allows the oscillationwavelength of the laser light which emerges from the light source 12 tobe controlled in a manner to correspond to the absorption wavelength λof the absorbing cell 8 and the waveform-stabilized light to exit to thediffraction grating 2.

In the rotation angle detection apparatus according to the secondembodiment, a calculation unit 6 decides, as an absorbing line, a pointat which the power of the diffraction light received by a referencelight receiving unit 3 becomes maximal and calculates the rotation angleof the diffraction grating in the same way as set out in conjunctionwith the above-mentioned first embodiment.

FIG. 3 is a schematic view showing the application, to the opticalspectrum analyzer, of the rotation angle detection apparatus shown inFIG. 1.

The optical spectrum analyzer includes not only the rotation angledetection apparatus but also a light receiving unit 16 for measurement,slit mechanism and switching unit 18.

Though in FIG. 3 the reference light from the waveform reference lightsource 1 and light to be measured are illustrated as being incident onthe diffraction grating 2 at different places, they are incident on thesame position, or a position very near to that position, in actualpractice.

The light receiving unit 16 for measurement receives a diffractedreplica of the to-be-measured light incident on the diffraction grating2.

The above-mentioned slit mechanism 17 includes a slit 17a on an opticalaxis between the diffraction grating 2 and the light receiving unit 16.

This slit is so provided as to narrow down the diffraction light andconduct it to the light receiving unit in a way to eliminate any excesslight portion, because the diffracted light from the diffraction grating2 is reflected in a given broadening range.

By making the slit width H of the slit mechanism 17 variable thenarrowing-down extent of the diffracted light from the diffractiongrating 2 is set to an optimal state to enhance a spectral resolution.

The switching unit 18 is comprised of a switch operated by, for example,an operator and, in order to prevent any interference between theto-be-measured light and the reference light, the ON/OFF switching ofthe waveform reference light source 1 is effected by the operation ofthe switch.

The switching unit 18 may be so configured as to allow any one of thereference light from the waveform reference light source 1 or theto-be-measured light to be incident on the diffraction grating 2.

If the reference light from the wavelength reference light source 1 andto-be-measured light are incident in a way to be shifted in a length orin a width direction, it is possible to omit the structure of theswitching unit 18.

A drive unit scans the diffraction grating 2 through a predeterminedangle range with a horizontal state as a reference and, when correctiondata calculated by the calculation unit 6 is input, rotationally drivesthe diffraction grating 2 through a rotation angle based on thecorrection data.

In the optical spectrum analyzer thus constructed, the diffractiongrating 2 is rotationally driven to an allocated position correspondingto the wavelength (the wavelength of the to-be-measured light) set bythe operator.

In this state, with the use of an absorption cell 8 having an absorptionline matched to the wavelength of the to-be-measured light, theswitching unit 18 is rendered ON to allow light which comes from thewavelength reference light source 1 to exit toward the absorption cell8.

The light having an absorption spectrum corresponding to only anabsorption line-existing waveform component is exited as transmittedlight from the absorption cell 8.

The diffraction grating 2, receiving the transmitted light, sendsdiffracted light so that it is received by the reference light receivingunit 3.

When the diffracted replica of the transmitted light having a spectrumabsorbing only the absorption line-exiting waveform component isreceived by the reference light receiving unit 3, then correction datacorresponding to an angle detection signal from an angle detectionsignal from an angle detector 4, that is, the angle detection signalallocated to the wavelength of reference light, though being omitted inillustration, on the basis of the absolute value of an absorption linewavelength at that time, is calculated as the rotation angle of thediffraction grating 2 as set out above.

The drive unit 5 rotatably drives the diffraction grating 2 through arotation angle range based on the calculated correction data.

This ends the correction of the rotation angle of the diffractiongrating 2 by the wavelength reference light source 1.

In this state, the switching unit 18 is rendered OFF, stopping theexiting of the light from the wavelength reference light source 1 andentering to-be-measured light into the diffraction grating 2.

The diffraction grating 2 receives the to-be-measured light and sendsdiffracted light 2 to that slit 17a in the slit mechanism 17 where it isnarrowed down. The narrowed-down signal is received by the lightreceiving unit 16 for measurement and the wavelength distribution of theto-be-measured light is detected based on the absolute value of thewavelength at that time.

By doing so, the optical spectrum analyzer shown in FIG. 3 suppress avariation in the absolute value of the wavelength of the diffractedlight resulting from a variation under the environment such as theatmospheric temperature, humidity and atmosphere and can enhance theaccuracy with which the wavelength is measured.

FIG. 4 is a schematic view showing the application, to a tunablewavelength light source, of the rotation angle detection apparatus shownin FIG. 1.

The tunable wavelength light source 21 includes not only the structureof the rotation angle detection apparatus but also a principal lightsource 22 and switching unit 23.

Though, in FIG. 4, reference light coming from a wavelength referencelight source 1 and light coming from the principal light source 22 areillustrated as being incident on a diffraction grating 2 at twodifferent positions, they are incident on the same position, or aposition near to that position, in actual practice.

The above-mentioned principal light source 22 is comprised of a laserdiode.

The light is reciprocably moved back and forth between one end face 22aof the principal light source 22 and the diffraction grating 2 locatedon that optical axis and resultant resonant light is output from theother end face 22b and, by doing so, an external cavity type tunablewavelength light source is realized.

The switching unit 23 is comprised of a switch operated by, for example,the operator and, in order to prevent interference between the lightfrom the principal light source 22 and the light from the wavelengthreference light source 1, the ON/OFF switching of the wavelengthreference light source 1 is effected by the operation of that switch.

The switching unit 23 may be so constructed as to enable any one of thelight from the wavelength reference light source or light from theprincipal light source 22 to be incident on the diffraction grating 2.

Further, if the reference light from the wavelength reference lightsource 1 and light from the principal light source 22 are made incidenton the diffraction grating 2 by shifting them in a length or in a widthdirection of the diffraction grating 2, then it is possible to omit theswitching unit 23.

A drive section 5 for scanning the diffraction grating through apredetermined angle range with a horizontal state as a referencerotationally drives the diffraction grating 2 through a rotation anglecorresponding to correction data when the correction data calculated bya calculation unit 6 is entered.

In the tunable wavelength light source thus arranged, first thediffraction grating 2 is rotated to a position (wavelength of theprincipal light source 22) allocated to a wave length set by theoperator.

With the use of an absorption cell 23 possessing an absorption linematched to the wavelength of light from the principal light source 22,the switching unit 23 is turned ON to allow light to be incident on theabsorption cell 8 from a light source 7 in the wavelength referencelight source 1.

The light possessing a spectrum absorbing only an absorptionline-existing wavelength component is sent as transmitted light from theabsorption cell 8 to the diffracted grating 2.

The diffracted light resulting from the reception of the transmittedlight is received by a reference light receiving unit 3.

When the diffracted replica of the transmitted light possessing thespectrum absorbing only the absorption line- existing wavelengthcomponent is received by the reference light receiving unit 3, thecalculating unit 6 calculates, as a rotation angle of the diffractiongrating 2, correction data corresponding to an angle detection signalfrom an angle detector 4, that is, a signal allocated to the wavelengthof the light from the wavelength reference light source 1, though beingomitted in illustration, on the basis of the absolute value of theabsorption line wavelength at that time.

The drive unit 5 rotationally drives the diffraction grating 2 throughan angle corresponding to the calculated correction data.

By doing so, the correction of the diffraction angle of the diffractiongrating 2 is finished by the wavelength reference source 1.

In this state, the switching unit 23 is turned OFF, stopping the exitingof the reference light from the waveform reference light source 1 andexiting light from one end face 22a of the principal light source 22 tothe diffraction grating 2.

The incident light from the one end face 22a of the principal lightsource 22 is reciprocated between the diffraction grating 2 and the oneend face 22a of the principal light source 22 and the resultant light isoutput from the other end face 22b of the principal light source 22.

By doing so, the tunable wavelength light source shown in FIG. 4suppresses a variation in the wavelength of the diffracted light fromthe diffraction grating 2 resulting from a variation in theenvironmental condition, such as the atmospheric temperature, humidityand pressure and it is, therefore, possible to enhance the accuracy withwhich the wavelength oscillates.

In the aspect of the respective embodiment, although the transmittedlight is incident on the diffraction grating 2 via the absorption cell 8and the correction of the diffraction angle is made by receiving only a1-order diffracted light (m=1), it is possible that, in additionthereto, the diffraction grating 2 is also rotated with the incidentlight intact. If, by doing so, the diffraction angle is corrected bysequentially receiving 2- and 3-order diffracted light, it is possibleto correct the rotation angle of the diffraction grating 2 moreaccurately in a broader range.

In the aspect of the respective embodiment, although the motor of thedrive unit 5 is driven by the correction data calculated by thecalculation unit 6, if a structure is an optical spectrum analyzer typeequipped with a display 25 as shown in FIG. 5, it may be possible tocorrect the graduations representing practical wavelength on a displayunit 25 at a display screen, as shown in FIGS. 6A, 6B, on the basis ofthe correction data calculated by a calculation unit 6.

That is, FIG. 6A shows the case where, instead of changing thosewavelengths λa, λb on the display screen, the display position of thespectra is so displayed as to be changed from a solid line to a brokenline as indicated on FIG. 6A.

FIG. 6B shows the case where the wavelengths λa, λb are changed to λa'(=λa ±Δλa), λb' (=λb ±Δλb) on the display screen.

The light spectrum analyzer shown in FIG. 5 uses a referencelight/measured light receiving unit 29 constituting a combination of thereference light receiving unit 3 and the light receiving unit 16, formeasurement, in FIG. 3.

The aspect of switchingly receiving diffracted light of the diffractiongrating 2 corresponding to the reference light or measured light, by areference light/measured light receiving unit 29, is the same as in FIG.3.

An electric signal from the reference light/measured light receivingunit 29 is converted, by an A/D converter 30, either illustrated oromitted in explanation in FIGS. 1 to 4, from an analog to a digitalsignal and input to a calculation unit (signal processing unit) 6.

In this case, a rotation angle setting section 28 equipped with thecalculation unit (signal processing unit) 6 has wavelength-versus-angledata initially stored in table format and, upon receipt of the value ofa wavelength set by the operator, sends, to a drive unit 5, angle datafor driving the drive unit to allow a rotation grating 2 to take arotation angle corresponding to the set wavelength value.

A data correction section 27 in the calculation unit 6 receives adigital signal from the A/D converter 30 at a time of selectingreference light from a switching unit 18 and calculates theabove-mentioned correction data.

The correction data calculated by the data correction unit 27 is sent toa display unit 25 and to a rotation angle setting unit 28.

When later-described spectrum data is displayed based on the correctiondata, a display unit 27 corrects graduations representing a substantialwavelength on a display screen as set out above.

The rotation angle setting section 28 corrects the above-mentionedwavelength-versus-angle data based on the correction data and effectsthe correction of the diffraction angle so that corrected angle data issent to the drive unit 5 to rotate the diffraction grating 2.

A spectrum processing unit 26 in the calculation unit 6 performsprocessing on a digital signal from the A/D converter 30, with switchingeffected by the switching unit 18 to to-be-measured light, and performsdisplay processing and sends data as spectrum data to the display unit25.

The calculation unit (signal processing unit) 6 as shown in FIG. 5 is soarranged as to incorporate its requisite parts in the calculation unit 6as shown in FIGS. 1 to 4.

As set out above, a first optical spectrometer apparatus according tothe present invention is characterized in that, in a light splittingdevice equipped with a rotatable diffraction grating 2 for receivingincident light, splitting it and outputting it, it comprises a lightsource 7 having a broader emission spectrum, an absorption cell 8 forreceiving light from the light source, allowing light of a specificwavelength to pass through and outputting it, and a reference lightreceiving unit 3 for receiving the light of the specific wavelengthsplit by the diffraction grating and converting it to an electricsignal.

Further, a second optical spectrometer apparatus is characterized inthat, in a light splitting device equipped with a rotatable diffractiongrating 2 for receiving incident light, splitting it and outputting it,it comprises a light reference light source 11 having an absorption cell8 for absorbing light of a specified wavelength, a laser diode 12oscillatable with a wavelength absorbing cell, a light splitting unit 13for allowing light which comes from the laser diode to be split into twolight beams, a waveform-controlling light receiving unit 14 for allowingthat first light beam which originates from the laser diode and whichcomes, as such, from the light splitting unit to be incident on theabsorbing cell, for receiving the first light beam, as a transmittedlight, from the absorption cell and for converting it to an electricsignal, and a control circuit 15 for receiving the electric signal fromthe wavelength-controlling light receiving unit and for controlling anoscillation wavelength of the laser diode so that the strength of thefirst light beam originating from the laser diode and transmittedthrough the absorption cell becomes constant, a light splitting unitequipped with a rotatable diffraction grating 2 for receiving thatsecond light beam originating from the laser diode and split, as such,by the light splitting unit and for splitting it, and a reference lightreceiving unit 3 for receiving the second light originating from thelaser diode and split by the light splitting unit and for converting itto an electric signal.

A third optical spectrometer apparatus is equipped with the structure ofthe first or the second optical spectrometer device and includes a lightreceiving unit, for measurement, for receiving diffracted light involvedby to-be-measured light incident on the diffraction grating, an angledetector 4 for detecting the rotation angle of the diffraction grating,a calculation unit 6 for calculating an amount of correction of therotation grating corresponding to the angle detector's signal given anallocated absolute wavelength of the to-be-measured light on the basisof the diffracted light of the above-mentioned absorption line-existingwavelength component received by the reference light receiving unit, anda drive unit for rotationally driving the diffraction grating through apredetermined angle range and, at the same time, rotationally drivingthe diffraction grating through only an angle corresponding to thecorrection amount.

A tunable wavelength light source according to the present invention isequipped with the first or the second optical spectrometer device andcomprises a principal light source 22 for reciprocating light, exitingfrom one end face 22a, back and forth relative to the diffractiongrating and for outputting resonant light from the other end face 22b,an angle detector 4 for detecting the rotation angle of the diffractiongrating, a calculation unit 6 for calculating a correcting amount of therotation angle of the diffraction grating corresponding to the angledetector's signal given an allocated wavelength of the principal lightsource on the basis of diffraction light of the above-mentionedabsorption line-existing wavelength component received by the referencelight receiving unit, and a drive unit 5 for rotationally driving thediffraction grating through a predetermined angle range and, at the sametime, rotationally driving the diffraction grating through only an anglecorresponding to the correction amount.

In the first or the second optical spectrometer apparatus, light exitingfrom the light source unit 7 utilizing the absorption line of a gas, orfrom the reference light source unit 11, is incident on the diffractiongrating 2.

The diffraction light receiving unit 3 receives its diffracted lightand, from the diffracted light of the absorption line-existingwavelength component, it is possible to know the rotation angle anddiffraction angle of the diffraction grating 2 at the wavelength at thattime.

In the above-mentioned third optical spectrometer apparatus, thediffraction grating 2 is rotationally driven in a range given anallocated wavelength of the to-be-measured light.

While the diffraction grating 2 is rotationally driven, light isincident from a wavelength reference light source 1, or the referencelight source unit 11, with the use of the absorption cell possessing anabsorption line matched to the wavelength of the to-be-measured light.

The diffracted light involved by the incident light at this time isreceived by the reference light receiving unit 3.

When the diffracted light of the absorption line-existing wavelengthcomponent is received by the reference light receiving unit 3, thecalculation unit 6 calculates, as the rotation angle of the diffractiongrating 2, the correction data corresponding to a signal of the angledetector 4, that is, the angle detector's signal given an allocatedwavelength of the to-be-measured light on the basis of the absolutevalue of the waveform of the absorption line at that time.

The drive unit 5 rotationally drives the diffraction grating 2 throughonly a rotation angle corresponding to the correction data calculated bythe calculation unit 6.

When, in this state, the to-be-measured light is incident on thediffraction grating 2, the diffracted light of the diffraction grating 2involved by the incident light is received by the light receiving unit16 for measurement and the wavelength distribution is detected as adistribution corresponding to the absolute value of the wavelength atthat time.

In the above-mentioned tunable wavelength light source, the diffractiongrating 2 is rotationally driven in a range given an allocatedwavelength of the principal light source 22.

While the diffraction grating 2 is rotationally driven, the light isincident on the absorption cell 8 from the waveform reference lightsource 1, or the reference light source unit 11, with the use of theabsorption cell 8 possessing the absorption line matched to thewavelength of the principal light source 22.

The diffracted light involved by the incident light at this time isreceived by the reference light receiving unit 3.

When the reference light receiving unit 3 receives the diffracted lightof the absorption line-existing wavelength component, the calculationunit 6 calculates, as the rotation angle of the diffraction grating 2,the correction data corresponding to the signal of the angle detector 4,that is, the signal given an allocated wavelength of the principal lightsource 22 on the basis of the absolute value of the wavelength of theabsorption line at this time.

The drive unit 5 rotationally drives the diffraction grating 2 throughonly an angle corresponding to the correction data calculated by thecalculation unit 6.

If, in this state, light is incident on the diffraction grating 2 fromone end face 22a of the principal light source 22, the incident lightreciprocates between the diffraction grating 2 and one end face 22a ofthe principal light source 22 and resonant light is output from theother end face 22b of the principal light source 22.

As explained above, according to the apparatus, according to the presentinvention, for detecting the rotation angle of the diffraction grating,it is possible to accurately detect the rotation angle of thediffraction grating, without affecting the environmental changes such asthe atmospheric temperature, humidity and pressure, and to improve theaccuracy of the absolute value of the wavelength of the diffracted lightof the diffraction grating and accurately also detect the rotation angleof the diffraction grating with respect to the wavelength at its time.It is also possible to actually measure the diffraction angle withoutrelying upon a theoretical value and to improve the accuracy of theabsolute wavelength of the diffracted light.

According to the optical spectrum analyzer to which the presentinvention is applied, it is possible to enhance the measuring accuracyof the wavelength by suppressing a variation in error of the absolutevalue of the wavelength of the diffracted light resulting from anenvironmental variation, such as the atmospheric temperature, humidityand pressure and to largely improve the existing wavelength accuracy.

Further, according to the tunable wavelength to which the presentinvention is applied, it is possible to enhance the accuracy of theoscillation wavelength by suppressing a variation in the absolute valueof the wavelength of the diffracted light due to the environmentalvariation such as the atmospheric temperature, humidity and pressure.

I claim:
 1. An apparatus for detecting a rotation angle of a diffractiongrating, which diffraction grating includes a rotatable diffractiongrating for receiving incident light and exiting split beams, and adrive unit for rotating the rotatable diffraction grating, the apparatusfurther comprising:an angle detecting unit for detecting a rotationangle of the rotatable diffraction grating; a light source unitincluding a light source and an absorption cell, said light source unitexiting, to the rotatable diffraction grating, reference light of awavelength range including a specified wavelength determined dependingupon the absorption cell, the absorption cell being comprised of a tubesealed with a gas having an absorption line absorbing the referencelight of a predetermined wavelength at all times without being affectedby a variation in an environmental condition; a reference lightreceiving unit for receiving a split reference light from the rotatablediffraction grating and converting the split reference light into anelectric signal; and a signal processing unit which (i) detects anextreme value of the electric signal from the reference light receivingunit, which electric signal is obtained while the rotatable diffractiongrating is rotated by the drive unit, (ii) receives a rotation angleoutput from the angle detecting unit when the extreme value is detected,and (iii) calculates a specified rotation angle corresponding to thespecified wavelength.
 2. The apparatus according to claim 1, wherein:thelight source unit comprises a waveform stabilizing light sourceincluding a light dividing unit, a light receiving unit for wavelengthcontrol, and a wavelength control circuit; the light dividing unitdivides light from the light source into a light component directed tothe diffraction grating and a light component directed to the absorptioncell; the light receiving unit for waveform control is arranged toreceive a light component which is directed to the absorption cell andwhich is transmitted through the absorption cell, and converts saidreceived light component into an electric signal; and the wavelengthcontrol circuit controls an oscillation wavelength of the light sourceto make constant a strength of the light which is transmitted throughthe absorption cell, based on the electric signal from the lightreceiving unit and, by doing so, a wavelength of a light componentdirected from the absorption cell to the diffraction grating is sostabilized as to be matched to an absorption wavelength of theabsorption cell.
 3. The apparatus according to claim 1, wherein exitlight from the absorption cell in the light source has a plurality ofabsorption lines and, through utilization of a different absorptivity ineach of the plurality of absorption lines, the signal processing unitdecides, from a variation in a split light level output from thediffraction grating, to which the split reference light received by thereference light receiving unit is matched to an absorption line of anywavelength, and calculates a corresponding rotation angle.
 4. Theapparatus according to claim 1, wherein the signal processing unitcomprises:a rotation angle setting unit having wavelength-versus-angledata, for receiving a set wavelength value and sending angle data fordriving the driving unit to allow the rotatable diffraction grating tobe set to a rotation angle corresponding to the set wavelength value;and data correcting means for correcting the wavelength-versus-angledata in the rotation angle setting unit with the use of the specifiedrotation angle calculated with respect to the specified wavelength. 5.The apparatus according to claim 4, wherein:the apparatus is used foranalyzing a spectrum of light to be measured; and the signal processingunit comprises means for supplying correction data from the datacorrection means to the drive unit in order to correct a diffractionangle of the diffraction grating prior to analyzing of the light to bemeasured.
 6. The apparatus according to claim 5, further comprising:ameasurement light receiving unit for receiving a split light of lightto-be-measured while the diffraction grating is being rotated inaccordance with the set wavelength value, and converting the split lightinto an electric signal; and a spectrum processing unit for outputtingthe electric signal output from the measurement light receiving unit andthe set wavelength value in a corresponding relation to each other. 7.The apparatus according to claim 5, further comprising:a measurementlight receiving unit for receiving a split light of light to-be-measuredwhile the diffraction grating is being rotated in accordance with theset wavelength, and converting the split light into an electric signal;and a spectrum processing unit for calculating an electric signal in theset wavelength value from the electric signal output from themeasurement light receiving unit based on waveform-versus-angle datacorrected by the data correcting means and outputting the set wavelengthvalue and electric signal in a corresponding relation to each other. 8.The apparatus according to claim 6, further comprising means forswitching between the reference light and the light to-be-measured. 9.The apparatus according to claim 6, wherein the reference lightreceiving unit and the measurement light receiving unit comprise asingle unit.
 10. The apparatus according to claim 4, wherein:therotation angle detection unit of the diffraction grating is used for anexternal cavity type tunable wavelength source for reciprocating andresonating light between the diffraction grating and a principal lightsource so as to output tunable wavelength light; and the signalprocessing unit comprises means for supplying correction data from thedata correction means to the drive unit in order to correct the rotationangle of the rotation grating prior to the outputting of the tunablewavelength light by the external cavity type tunable wavelength lightsource.
 11. The apparatus according to claim 10, further comprisingmeans for switching between the light source unit and the principallight source.
 12. The apparatus according to claim 7, further comprisingmeans for switching between the reference light and the lightto-be-measured.
 13. The apparatus according to claim 7, wherein thereference light receiving unit and the measurement light receiving unitcomprise a single unit.